Substrates that demand high expansion with good chemical durability are often manufactured from optical glasses. Optical glasses may be employed in various applications, such as substrates for thin-film interference filters used in fiber optic systems. Generally, these interference filters are fabricated from multiple layers of conducting and insulating materials or films that together result in a filter that passes only a narrow bandwidth of incident radiation. Such filters are described, for example, in Optical Filter Design and Analysisxe2x80x94A Signal Processing Approach by Christie K. Madsen and Jian H. Zhao published by John Wiley and Sons, 1999.
In one particular application, there is a strong demand for a glass substrate capable of being incorporated into an interference filter for wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) applications. Thin-film interference filters for WDM and DWDM applications have high requirements in terms of the narrow bandwidth of light transmittal (Introduction to DWDM Technology by Stamatios V. Kartalopoulos, published by IEEE Press, 2000). Such bandwidths are expressed as a width in passed frequency, typically 200 GHz, 100 GHz, 50 GHz, or less, with smaller values indicating a narrower bandwidth of transmission. For example, a 100 GHz filter within the 1.5 xcexcm telecommunications band corresponds to a wavelength spread of 0.8 nm; and, a 50 GHz filter within the same 1.5 xcexcm telecommunications band corresponds to a wavelength spread of 0.4 nm. These filters preferably have bandwidths of less than 200 GHz pass frequency in the 1.5 xcexcm wavelength region. An optical designer can fabricate useful telecommunications modules using such filters. For example, an optical demultiplexer can be constructed using a multitude of such thin-film interference filters, each one of which separates out a particular wavelength of interest.
Most desirably, the substrate is characterized by high transmission in the near IR where DWDM systems operate, i.e., wavelengths at or near 1.5 xcexcm, a refractive index value at 587.6 nm, nd, of between 1.50 and 1.70, and a high transformation temperature, Tg, exceeding 350xc2x0 C., most preferably exceeding 400xc2x0 C. High transmission at 1.51 xcexcm is characterized by a value of digital transmittance, including Fresnel reflection loss, exceeding 88%, more preferably equivalent to or exceeding 90% at 1.5 xcexcm through a 1.0 mm thickness. Preferably, these filters have minimal wavelength drift with change in temperature. Glass substrates with high thermal expansion, CTE, and high values of Young""s modulus, E, allow for decreased amounts of thermally-induced drift (dxcex/dT) in the transmission wavelengths of interest, e.g., 1450-1620 nm, 1480-1620 nm, and 1450-1550 nm. A particularly desirable range of thermal expansion values is from 90 to 140xc3x9710xe2x88x927/xc2x0 C., particularly 110 to 140xc3x9710xe2x88x927/xc2x0 C., over a temperature range of xe2x88x9230xc2x0 C. to +70xc2x0 C. coupled with a Young""s modulus  greater than 80 GPa. More preferably, the thermal expansion should lie in the range of 100 to 130xc3x9710xe2x88x927/xc2x0 C. over the same temperature range in combination with a Young""s modulus value  greater than 85 GPa.
Such narrow bandwidths are highly demanding and difficult to achieve and push the limits of available coating technology. Consequently, the substrate properties are becoming more demanding, and the advanced coating industry desires to have new substrate glasses available that offer enhanced or optimized properties for applications at less than 200 GHz bandwidth range.
Thus, a desired embodiment of the invention is a glass making available an interference filter for a fiber optic system including a substrate and a film coating the substrate. Typically, the substrate is coated with a series of layers of differing materials having properties, e.g., indices of refraction, producing interference effects achieving the desired wavelength transmission spectrum. Fiber optic systems comprise one or more light sources, fiber optic transmission components, filters and end use components, e.g., detection, amplifiers, etc. Glasses of the invention and their properties are described in the following tables:
RE=rare earth ions, excluding La, that do not impart unacceptable absorption at the wavelength of interest (e.g., 1450-1550 nm, especially 1480-1620 nm), i.e., do not degrade T overall beyond the numbers given above. As a more preferred example of acceptable absorption, RE allows for an internal transmission of  greater than 0.99 for a 1 mm thickness sample, thereby allowing for an insertion loss of  less than 0.9 dB. Nd and Ho are non-limiting examples of rare earth ions that may be used in the current application.
Without being bound by theory, it is believed that the individual components of the glasses affect certain properties. It is believed that in glasses of the present invention SiO2 and GeO2, both are network formers and Y2O3 and La2O3 are intermediates that do participate as network formers. Na2O is a network modifier that typically affects index, expansion, and transformation temperature. Li2O is a network modifier that affects index expansion, transformation temperature, Young""s modulus, and thermal conductivity. MgO is a network modifier that affects index, expansion, transformation temperature, Young""s modulus, and thermal conductivity. Sc2O3 and other rare earth oxides in the prescribed amounts can be directly substituted for Y2O3 and La2O3. The addition of TiO2 and/or ZrO2 to the glass helps maintain durability.
Expansion and Young""s modulus are properties that are normally inversely proportional to each other in glasses in that as one property is raised through compositional adjustments, the other is lowered (Glass, by Horst Scholze, 1991, published by Springer-Verlag). Surprisingly, the introduction of Li2O and/or MgO in glasses of the present invention causes the above properties to become proportional to each other so that both can be raised together as needed to produce a stable glass substrate with the required properties of high expansion and high Young""s modulus.
The substrates of the present invention may be made by conventional glass melting techniques. Raw materials can be melted in platinum crucibles and held at temperatures around 1400xc2x0 C. for up to five hours.
The interference filter for a fiber optic system also includes at least one film desirably in the form of a layer. Such films can be selected from SiO2, Ta2O5, HfO2, etc. These can be applied by commercially available standard ion beam deposition systems such as the SPECTOR(copyright) system available from Ion Tech, Inc. of Fort Collins, Colo., or the Advanced Plasma Source 1104 System from Leybold Optics of Hanau, Germany. In addition to being particularly useful for DWDM filters, these glasses are also exceptionally useful as high expansion glasses for fabrication of hybrid structures that demand a high expansion glass with good chemical durability, e.g., for the purposes of longwave pass filters, polarizing components, band pass filters, etc.
Preferred embodiments also include a glass comprising:
and the above glass preferably having the following properties:
as well as glass comprising:
the above glass preferably having the following properties:
and a glass comprising:
the above glass preferably having the following properties:
Preferred embodiments also include an interference filter comprising a glass substrate having at least two interference layers coated thereon, wherein the glass substrate comprises one of the compositional spaces above.
Preferred embodiments also include a fiber optic system comprising a light source, a fiber optic transmission component, a receiver of transmitted radiation and an interference filter comprising a glass substrate having at least two interference layers coated thereon, said glass substrate comprising one of the glass spaces above.
Moreover, preferred embodiments include a process for making glasses according to the invention comprising melting raw materials corresponding to oxides in the glass, refining a resultant glass melt, casting the melt in a mold and optionally annealing, or casting into a mold a glass melt produced from raw materials corresponding to oxides in the glass.
Additionally, preferred embodiments include a demultiplexing optical component comprising the above interference filter and a method of demultiplexing, comprising passing an optical signal of multiple wavelengths through a demultiplexing optical component above, whereby one or more wavelengths of interest are separated.
Finally, preferred embodiments include a process for making an interference filter comprising coating any glass described above.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following example, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by mole.
The entire disclosures of all applications, patents and publications, cited above or below including provisional applications number 60/259,706 filed Jan. 5, 2001, and 60/317,493 filed Sep. 7, 2001, are hereby incorporated by reference.